This invention relates to a control system for an ultraviolet light source, to a method of controlling a microwave energisable ultraviolet bulb and to apparatus for emitting ultraviolet radiation.
It is known that microwave-induced plasmas using a mixture of mercury mixed with elements such as iron, gallium, lead and in an inert gas, such as Ar, produce light, a large proportion of which is in the UV spectrum (320-445 nm).
Such a plasma may be contained in a transparent envelope which in practice is usually made from quartz. Striking of the plasma is made easier by evacuating the envelope and maintaining it at a lower pressure than atmospheric pressure (typically 10 mbar) prior to the plasma being struck. Once struck, energy is absorbed by the plasma and UV radiation is emitted via the UV-transparent quartz envelope.
Various methods of coupling the microwave energy to the plasma are known. For example, the bulb may be placed in a resonant cavity or be directly coupled to a microwave source using a transmission line such as a co-axial cable, or waveguide. Sometimes the addition of a tungsten or similar wire in the bulb envelope is used to aid striking.
Different UV lamp systems are currently available. Low power systems (typically up to 167 w/m rf input @ 20 mm envelope diameter) produce a xe2x80x9clow pressurexe2x80x9d spectral output, with peak output at UVC wavelengths (typically 254 nm). Medium pressure systems (typically 6.67 kw/m @ 20 mm dia) produce a xe2x80x9cmedium pressurexe2x80x9d spectral output with peak output at UVA wavelengths (typically 365 nm).
Hitherto, it has usually been difficult to predict the power densities of different wavelengths of ultraviolet radiation from microwave energised bulbs based on the input power levels because of wide variations in RF coupling into the bulb and because of differing bulb dimensions. This is a significant problem in applications where particular portions of the UV spectrum (commonly designated UVA, UVB, UVC and UW) are desired to be emitted in particular power levels. For example in curing or germicidal applications, particular energy levels (often expressed as joules per square centimeter) of radiation need to be applied to an article. This has conventionally been carried out by making power measurements and then assuming that these measurements will hold good throughout the duration of bulb operation. With a known power level, the exposure or energy per unit area may be controlled by controlling the duration of exposure.
However a significant limitation of this approach is that in practice, the power output of the bulb varies over time.
In accordance with the invention there is provided a control system for an ultraviolet light source comprising a controller having spectral input means arranged to receive an input signal representative of the spectral power distribution of an ultraviolet light source, and control output means arranged to cause an adjustment in the energy input into the ultraviolet light source and/or to cause a change in the heat energy extracted from the ultraviolet light source responsive to the signal received at the spectral input means.
In another aspect of the invention there is provided a control a system of the type defined in the preceding paragraph in which the controller is arranged to cause a reduction in the energy input into the ultraviolet light source and/or to cause an increase in the heat energy extracted from the ultraviolet light source when the signal received at the spectral input means indicates a ratio of power in the UVC spectrum against the power of another predetermined portion of the UV spectrum or the whole of the UV spectrum which is below a predetermined threshold.
In a method aspect, the invention provides a method of controlling a microwave energised ultraviolet bulb comprising periodically measuring the spectral power density of the bulb output, deriving a measure of the power density in a first predetermined portion of the UV spectrum relative to the power density of a second predetermined portion of the UV spectrum which is overlapping or non-overlapping with the first portion, and controlling the bulb temperature by adjusting the RF output power of a microwave source coupled to the bulb and/or adjusting the thermal energy extracted from the bulb responsive to the derived measure, whereby the UV output of the bulb as a function of microwave energy input is optimised.
In a further apparatus aspect there is provided apparatus for emitting ultraviolet radiation comprising a source of microwave energy, a microwave energised ultraviolet bulb coupled to the microwave source, an ultraviolet transducer arranged to measure the spectral power density of ultraviolet light output by the bulb and a controller arranged to receive the output of the ultraviolet transducer, to analyse the power density of a first part of the output spectrum of the bulb relative to a second overlapping or non-overlapping of the part of the output spectrum of the bulb and to adjust the temperature of the bulb responsive to the relative power densities of the first and second portions of the bulb output spectrum.
As will be explained below, by monitoring the proportions, for example, of UVA and UVC emitted by a UV bulb, it is possible to operate the bulb atoptimum efficiency.